Steels have been developed having a very favorable yield strength/tensile strength ratio during forming operations.
Their strengthening capacity is very high, which allows a good distribution of deformations in the case of a collision and allows obtaining a significantly higher yield strength on parts after forming. It is thus possible to produce parts that are as complex as with traditional steels, but with higher mechanical properties, which allows a decrease in thickness to keep identical functional specifications. In this way, these steels have an effective response to the lightening and safety requirements of vehicles.
In particular, steels whose structure comprises martensite, optionally bainite, within a ferritic matrix, have experienced major development, since they combine high strength with significant deformation possibilities.
The recent requirements of lightening and reducing energy consumption have led to increased demand for very high-strength steels, the tensile strength TS of which is greater than 1180 MPa.
Aside from this level of strength, these steels must have a good ductility, good weldability and good coatability, in particular good suitability for continuous galvanization by hardening.
These steels must also have a high yield strength and elongation at break, as well as a good formability.
Indeed, certain automobile parts are manufactured through forming operations combining different deformation modes. Certain microstructural features of the steel may prove to be well suited to one deformation mode, but unfavorable with respect to another mode. Certain portions of the parts must have a high tensile strength and/or a good bendability and/or a good cut-edge formability.
This cut-edge formability is assessed by determining a hole expansion ratio, denoted Ac %. This hole expansion ratio measures the suitability of the steel for undergoing an expansion during cold pressing, and therefore provides an assessment of the formability of the steel in this deformation mode.
The hole expansion ratio may be assessed as follows: after producing a hole by cutting in a steel sheet, a frustoconical tool is used so as to expand the edges of that hole. During this operation, it is possible to observe early damage near the edges of the hole during the expansion, this damage beginning on second phase particles or at the interfaces between the different microstructural components in the steel.
Described in standard ISO 16630:2009, the hole expansion method consists of measuring the initial diameter Di of the hole before pressing, then the final diameter Df of the hole after pressing, determined when cracks are observed crossing through the thickness of the steel sheet on the edges of the hole. The suitability for hole expansion Ac % is then determined using the following formula:
      Ac    ⁢                  ⁢    %    =      100    *                                        D            f                    -                      D            i                                    D          i                    .      
Ac % therefore makes it possible to quantify the suitability of a steel sheet to withstand pressing at a cut-out orifice. According to this method, the initial diameter is 10 millimeters.
According to documents US 2012/0312433 A1 and US 2012/132327 A1, steels are known having a tensile strength TS greater than 1180 MPa. Nevertheless, this tensile strength is obtained to the detriment of the formability and weldability.
Furthermore, according to documents US 2013/0209833 A1, US 2011/0048589 A1, US 2011/0168300 A1 and WO 2013/144376 A1, steels are known having a high tensile strength that may exceed 1000 MPa, but not simultaneously having a satisfactory formability and weldability.